DESCRIPTION (Adapted from applicants abstract): The overall goal of this research proposal is to define the role of non-MHC encoded genes in tissue graft rejection/alloreactivity. There is growing evidence that although MHC molecules mediate these processes, the actual ligands recognized by effector T cells may include self-peptides which constitutively occupy the MHC binding site. This model offers an explanation for the observation in mice that the frequency of alloreactive T remains high (approximately 1 percent) even if the genetic differences present involve only two or three amino acid substitutions in one MHC (H-2 in mice) molecule. The model however, raises several new questions, namely: 1) What fraction of alloreactive T cells recognize primary sequence differences versus peptide contributed H-2 epitopes? 2) Which self peptides can serve as co-ligands for alloreactive T cells? and 3) Which enzymes are involved in the processing/presentation of self-peptides as co-ligands for allostimulation? These questions will be addressed in the well-characterized murine model--the I-Abm12 mutation. Specific Aim 1 is to determine the fraction of 200 available I-Abm12 reactive T cell hybrids whose stimulation requires I-Abm12 but appears independent of other genes expressed by the presenting cell. In this way, hybrids requiring non-MHC encoded genes for allostimulation will be identified which can then be used in Specific Aims 2 and 3 to detect L cell transfectants expressing the implicated genes. The transfectants will then be used as reagents to clone the genes involved and to determine the biochemical processes required for allostimulation.